The present invention relates generally to formation of abradable tips on turbine blades and to articles formed by the methods. More particularly, it relates to a method for forming a particle which is adapted to incorporation into the tips of turbine blades to provide abradable properties to the blade tips and to the article formed by the method.
The use of turbine blades with abrasive materials in the tip is known. One reason for such products and the use of such products is in developing a clearance between the end of the turbine blade and the housing in which the blades rotate.
It is known that the gas which is compressed with in a compression zone by operation of the compression stage of a gas turbine seeks escape from the zone through any available opening in the manner which is normal and characteristic for gaseous materials. One opening is at the space in the clearance between the edge of a turbine blade and the housing in which the blade rotates. It has been found advantageous and desirable to keep the clearance between the blade edge and the housing as small as is feasible. This is a difficult feat for an apparatus in which the blades rotate on a shaft within an enclosing housing at about 12,000 revolutions per minute.
Very close tolerances between the end of the blade and the enclosing housing are desirable but the maintenance of such a small clearance can be difficult as stress is applied to the various parts of the engine and slight movement of the parts takes place.
One way in which a close clearance has been maintained is by the incorporation of abrasive material in the blade end so that if it makes slight contact with the enclosing housing it is able to abrade and wear away the material at the place where contact was made.
There are a number of characteristics which are desirable in an abrasive blade end and in the abrasive material which is incorporated into the blade end. One such characteristic is that the abrasive material be resistant to oxidation in the engine environment. Another desirable characteristic is that the material incorporated in the blade end be significantly abrasive. Another characteristic is that the abrasive material incorporated into the blade end be chemically compatible with the metal of the blade in the solid state form during the full useful life of the blade itself. An additional requirement is that the particle which is to perform the abrasive function be capable of being incorporated into the blade.
In this last regard, one of the most effective manners in which such particles can be incorporated into the blade end is by causing a melting of the turbine blade tip, as by laser melting, and by incorporating the material to serve the abrasive function into the melted end of the blade. In order for an abrasive particle to be incorporated into the melted end of the turbine blade, the density of the particle must be greater than about 8 grams per cubic centimeter. If the density is lower then the particles will float on the top of the melt formed at the tip of a turbine blade and will not be incorporated in the blade tip.
A material such as tantalum carbide has a desirable set of properties in that it has a density of greater than 8 grams per cubic centimeter and serves usefully as an abrasive. Tantalum carbide particles have in fact been incorporated into the tips of turbine blades to serve as abrasive material. However, although they meet many of the criteria for such materials, they do not have sufficient oxidation resistance. A survey of readily available abrasive materials which have suitable thermal stability and oxidation resistance indicates that most have densities lower than the 8 gram per cubic centimeter which is required for direct incorporation into the melted end of a turbine blade tip. There are some materials which are available and might serve as abrasive materials in the tips of turbine blades and which meet the technical requirements which are outlined above. These materials are described in a publication of R. L. Fleischer, entitled "High Temperature, High Strength Materials--An Overview", July 1985. However, the use of these materials would require a considerable research effort to establish the properties of the materials and the feasibility of their being incorporated and operating suitably in a turbine blade tip.
Rather than dealing with some of these materials which are not well known or well characterized, it is recognized that it would be highly desirable to be able to use more conventional and better characterized abrasive materials. This is particularly true of materials which are well known to have fine abrasive properties and which nevertheless have the oxidation resistant in addition to the set of required properties which are outlined above. One such abrasive material is aluminum oxide. This material meets all of the required criteria for use in a turbine blade tip, except for the density criteria. The aluminum oxide has a density of 3.96 grams per cubic centimeter and thus has only about half of the density required for use in a turbine blade tip.